Method of erecting coaxial electric feeder



Dec. 2, 1958 E. c. CORK ET AL METHOD OF ERECTING COAXIAL ELECTRIC FEEDEROriginal Filed- June 18, 1948 lKhH l1 1 United States Patent METHOD OFERECTING COAXIAL ELECTRIC FEEDER Edward Cecil Cork, Ealing, London, andMichael Bowman-Manifold, Four Marks, Alton, England, assignors toElectric & Musical Industries Limited, Hayes, Middlesex, England, acompany of Great Britain Original application June 18, 1948, Serial No.33,866, now Patent No. 2,701,864, dated February 8, 1955. Divided andthis application November 4, 1954, Serial No. 466,900

Claims priority, application Great Britain June 21, 1947 1 Claim. (Cl.29-433) This invention pertains to radio frequency feeders, and moreparticularly to an improved method for erecting a vertical concentricline feeder for conveying radio frequency energy to an elevated point,such as the top of a transmitting tower, and is a divisional applicationfrom our co-pending application Serial No. 33,866, now Patent No.2,701,864 of February 8, 1955.

A field in which such vertical feeders are of special utility is that oftelevision transmission. In order to obtain a desirable range ofcoverage in the frequencies involved, the transmitting towers oftelevision stations may well rise to heights of 800 feet or more abovegnound level, the antenna itself being carried at the top of such atower. Both the height of the tower and the relatively high values ofthe carrier frequency introduce problems not encountered in eitherhorizontal or ver tical concentric line feeders of more modest length.

Conventional prior art feeders of the vertical type have heretofore beencomposed of a plurality of sections; that is to say, the inner conductorhas been sectionalised and the outer conductor has also beensectionalised, since feeders of the type to which the invention relatescannot be constructed as composite units as is usual with the type offeeder employed in television receivers where the inner conductor issurrounded by solid insulation and the outer conductor is usually in theform of a braiding covering the insulation. Feeders of the kind to whichthe invention relates have frequently an outer conductor having anexternal diameter of 5 inches or more which is air spaced from the innerconductor and when these feeders have to be taken up a tower of 800 feetor more the mass of the feeder is considerable, since in general theinner and outer conductor sections are composed of copper. It will beappreciated, therefore, that whilst for comparatively small heights aconcentric vertical feeder can be constructed, as is known in the priorart and the whole feeder supported at the bottom of the tower, whencomparatively larger lengths of feeder are required then it is foundthat if the feeder is supported at its lower end the mechanical loadingon the sections of the lower endof the feeder tend to cause buckling ofthe outer and centre conductors of the feeder, introducing electricalirregularities. Furthermore, the construction of a feeder even ifpossible in accordance with the prior art methods necessitates the useof considerable labour.

It is accordingly a principal object of the present invention to providea method of erecting or assembling an improved vertical concentric linefeeder of the above type which will be relatively free from electricalirregularities and losses which are characteristic of conventionalfeeders known to the art, and which can be assembled and installed withrelative ease and at relatively low cost.

A further object of the invention is to provide a method of constructinga feeder of this general type in which the necessity for electricaltesting and correction of individual sections, as the installationproceeds, is minimised or even completely eliminated, The necessity forsuch tests and corrections in the vertical assembly of long feeders ofprior types constitutes a major objection to the use of such designs.The importance of the present invention in eliminating or reducing suchoperations can readily be appreciated when it is considered that theerection must often be accomplished under adverse conditions of weatherand the like, and that any increase in erection time due to necessaryelectrical testing represents increased labour not only of theradiotechnicians but of the riggers handling the mechanical aspects ofthe installation. Still another object of the invention is to provide amethod of erecting a vertical concentric line feeder in which both outerand inner conductors are assembled by a method which results in both theouter and inner conductors being only in tension; in other Words, one inwhich both the inner and outer conductors are suspended from the upperend of the transmitting tower. The satisfactory accomplishment of thisobject provides a feeder which is inherently free from mechanicalbuckling or loss of concentricity, regardless of the vertical length ofsuch feeder. The above and other objects of the invention will best beunderstood by referring now to the following detailed specification of apreferred embodiment thereof, given by way of example and not oflimitation, and taken in connection with the appended drawings, in whichFigure 1 of the drawings illustrates the general construction of feederwhich is assembled in accordance with the invention. The referencenumeral 1 indicates the central conductor of a concentric line aerialfeeder and the reference numeral 2 the outer conductor thereof. Thecentral conductor of the feeder is suspended at its upper end from atower or other supporting structure to which the aerial proper ismounted, a platform, girder or some other suitable support for theaerial being indicated at 3. The central conductor is attached at itsupper end to a trunnion 4 carried by bearings 5 bolted or otherwisesecured to the member 3 and depending from the trunnion 4 is a stirrup 6to which the outer conductor 2 of the feeder is connected and suspendedfrom its upper end. The central conductor of the feeder may, forexample, be about 1'' in diameter and the outer conductor may be about5" in diameter. The central conductor is in the form of a wire rope ofthe so-called locked-coil type or it may comprise an inner core of wirerope provided with a covering of insulating material such as polytheneprovided with an outer sheath of copper tape. A locked-coil type of ropeis one in which strands of the rope have such cross-sectional shape asto interlock with one another so that when the rope is tensioned theinterlocking strands serve to prevent any substantial twisting of therope.

The lower end of the central conductor is preferably mechanically loadedeither by the provision of a spring or weight which latter may belocated in a dash-pot so as to maintain the central conductor taut. Themechanical loading is indicated by the reference numeral 7.

The outer conductor of the feeder comprises sections of copper tubingprovided with annular end flanges as shown by means of which thesections are bolted or otherwise secured together end-to-end and atconvenient positions along' the length of the outer conductor junctionboxes 8 and 9 are provided housing an aerial lead 14 and a transmitterlead 11 respectively. At the upper end of the feeder a short-circuitingpiston 12 is provided whilst a similar short-circuiting piston 13 isalso provided at its lower end, these pistons being for tuning andmatching purposes.

In assembling the feeder in accordance with the inven- Patented Dec. 2,1958 ductor and junction boxes are passed successively over the lowerend of the suspended'central conductor, hauled to appropriatepositions-and'thenbolted toone another.

The central conductor is spaced inthe'junction box 8 by aninsulating'sleevel l-and the lower end of the central conductor issimilarly located in the junction box 9' by an insulating sleeve 15. Thespacing of the central conductor along the major length of the outerconductor can be effected-by the provision of insulators disposed alongthe length of said'central-conductor and suitably spaced apart by adistanceof, for example, one-eighth of a wavelength at-the-operatingfrequency of the aerial. In the embodiment shownin Figure. 1, two ofsuch insulators are shown,- such-insulators-comprising insulating arms16 projecting radially from the central conductor and carried byconducting supports 17 of any suitable form. The insulating arms 16 maybe inserted in the apertures in the supports 17 as the sections of theouter conductor are assembled. Preferably, as shown, the pairs of arms16 are staggered rotationally along the length of the central conductor.If desired, insulators of the bead type may be used.

At the lower end of the outer conductor an expansion joint is providedwhich is of any suitable construction, and as shown in Figure 1 saidjoint comprises a metal bellows 18 secured to adjacent sections of theouter conductor. An expansion joint is also provided asstciated with thecentral conductor at its lower end, said joint comprising a metalbellows 19 attached at one end to the inner conductor and at its otherend to a sleeve 20 to which the transmitter lead 11 is also connected,the sleeve having a tubular extension 21 through which the innerconductor passes in electrical contact therewith. The junction box 9 onthe lower end-of the outer conductor of the feeder may be carried by asupporting platform or structure 22. The feeder is preferablyhermetically closed'and may beprovided with a filling of a suitable gassuch as nitrogen to reduce sparking and to avoid ingress of moisture.

The aerial lead 10and the transmitter lead 11 may be be tapped into thecentral conductor at suitable points along the length thereof, thepoints being suitably chosen so as to provide the required impedancematching between the aerial and transmitter leads. The outer conductorof the feeder will, of course, be provided with suitable branches toenclose the aerial and the transmitter leads in known manner, thesebranches being also suitably hermetically closed.

It will be appreciated that due to the construction described and themethod of assembly the feeder can be assembled far more rapidly than theknown prior art feeders and that buckling of the central conductor isavoided and, furthermore, due to the suspension of the outer conductorfrom its upper end buckling thereof is also avoided.

Figure 2 of the drawings illustrates diagrammatically a modifiedconstruction according to the invention in which the central conductoris suspended at its upper end from a transverse conducting support inthe form of a trunnion 23 which is supported at its ends on girders 24,the trunnion 23 being enclosed by sections 25 of outer conductorwhich'enclose the trunnion on each side of the central conductor to anextent efiectively equal to one-quarter of the operating wavelength ofthe aerial so that said trunnion has negligible efiect on the electricalcharacteristics of the feeder. A similar construction is provided at thelowerend of the feeder and in this case the central conductor passesthrough a sleeve 26 provided in the supporting trunnion 27 and is inelectrical contact therewith. The central conductor at its lower endissh'ort-circuited to the outer conductor by a piston 28.

What we claim'is:

The method of constructing in place, for a transmitting station or thelike, a vertical coaxial electrical feeder whose vertical length ismanytimes its diameter, comprising the steps of hauling to the upper part ofa tall supporting structure one end of a wire rope constituting theinner conductor of the feeder, suspending said inner conductor from theupper part of'said'structure with the remainder of said inner conductordepending therefrom, threading thelower end of said inner conductorthrough a tubular section ofouter conductor, hauling said tubularsection to the upperpart'of saidsupporting structure and, suspending ittherefrom, successively threading the lower end ofthe 11I1I16IC011d116t01 through additional tubular outer conductor sections, haulingthem upward and attachingthemin'succession to the'first section and toone another to'complete the outer conductor, and inserting spacing meansbetween the successive tubular sections and the inner conductor, toprovide a coaxial feeder whose conductors are supported substantiallyentirely in tension from the upper part of the supporting structure.

References Cited in the file of this patent UNITED STATES PATENTS1,463,015 Guay July 24, 1923 1,559,807 Thaden Nov. 3, 1925 2,355,833Bertalan Aug. 15, 1944 2,637,895 Blaton May 12, 1953 2,680,901 KaiserJune 15, 1954 2,701,864 Cork Feb. 8, 1955

